The inventors have been involved in the development of a detector for measuring the phase modulation of a hologram image. The hologram images are sensed by a solid state imager, which converts the hologram images to electronic data. The interferometer being developed promises to improve the sensitivity of the phase modulation measurement of optical wavefronts by 10 to 20 dB over prior art techniques. To realize this improvement, new light sensing and signal processing approaches have had to be followed that allow hologram images to be processed at high resolution and speed, i.e., 1000.times.1000 pixels at a 100 kHz frame-rate.
In an optical interferometer using a solid state imager to sense holograms, successive frames of pixels are correlated on a pixel-by-pixel basis to determine phase changes in the light illuminating that particular pixel, and the individual correlation results are then summed. In the summation process phase change information is correlated and thus sums in a scalar addition process, while noise is substantially uncorrelated and thus sums in a vector addition process. Consequently, in the summation result signal-to-noise ratio is improved by a factor nominally equal to the square root of the number of pixels being summed. The summation may be part of an averaging wherein the summation is subsequently divided by the number of samples that were summed.
U.S. Pat. No. 4,780,605 issued Oct. 25, 1988 to J. J. Tiemann, entitled "COHERENT LIGHT PHASE DETECTING FOCAL PLANE CHARGE-TRANSFER-DEVICE", assigned to General Electric Company and incorporated herein by reference describes detecting the phase of coherent light by interferometric procedures using a solid-state imager that has a planar array of charge-transfer devices (CTDs) constructed on a substrate of semiconductive material. The planar array senses the interference or "fringe" pattern between a coherent optical carrier and an optical signal generated through phase-modulation of a coherent optical carrier of similar frequency, but not necessarily the same phase. This pattern is a hologram. Each CTD in the focal-plane array has a first charge storage region associated therewith for storing charge generated by a reference-phase coherent optical carrier and has a second charge storage region for storing charge generated by an oppositely phased coherent optical carrier of the same wavelength. The stored charges stored in the first and second charge storage regions are differentially sensed to remove direct-current pedestal from the in-phase signals commutated from the solid-state imager. Each CTD has a third charge storage region associated therewith for storing charge generated by a quadrature-leading-phase coherent optical carrier and has a fourth charge storage region for storing charge generated by a quadrature-lagging-phase coherent optical carrier of the same wavelength. The stored charges stored in the third and fourth charge storage regions are differentially sensed to remove direct-current pedestal from the quadrature signals commutated from the solid-state imager. Utilizing scanning generators located on the imager substrate, the picture elements (or "pixels") are successively scanned to provide both a time-division-multiplexed in-phase output signal and a time-division-multiplexed quadrature output signal from the imager. Reading out in-phase and quadrature signals from the same pixels concurrently eliminates the need for long-time individual sample storage. The in-phase and quadrature signals for each pixel are processed in accordance with trigonometric algorithms to obtain hologram pixel phase variations. The hologram pixel phase variations are summed in a correlation procedure to increase the signal-to-noise ratio of the optical phase detection results. In a variant of the U.S. Pat. No. 4,780,605 apparatus charges are stored for only one phase of reference coherent optical carrier and for only one phase of quadrature coherent optical carrier and direct components are suppressed by resampling procedures. A practical difficulty encountered with implementing the U.S. Pat. No. 4,780,605 interferometer or variants thereof is the need for accurately matching the amplitudes of the various-phase pulses of coherent light used for charging respective ones of the pair of charge storage regions associated with each CTD.
J. Chovan, W. E. Engeler, W. Penn and J. J. Tiemann in allowed U.S. patent application Ser. No. 338,881 filed Apr. 17, 1989, entitled "ELECTRONIC HOLOGRAPHIC APPARATUS", assigned to General Electric Company and incorporated herein by reference describe interferometers in which a single pulse of coherent light is used for generating both in-phase and quadrature signals for each pixel. To do this the spatial frequency of the pixels as defined by the spacing of the CTDs in parallel rows normal to the fringe pattern is arranged to be four times the spatial frequency of the fringe pattern. Utilizing scanning generators located on the imager substrate, the in-phase and quadrature-phase responses of successively scanned pixels are extracted on a time-division-multiplexed basis to be separated by comb filtering procedures taking place mostly outside the solid state imager.
To meet the requirements of the new optical interferometer small phase-shifts in a hologram image must be detected and processed at extremely high rates. Currently, solid-state imagers are viewed providing the best electronic cameras for real-time hologram image detection. However, system throughput would be limited by the relatively slow rate at which data can be read-out from existing solid-state imagers in which pixel locations are successively scanned.
A solid-state imager of the most usual type reads out individual picture-element (or "pixel") information on a time-division-multiplexed basis through a single output port. The maximum read-out rate on an output port is typically 20M pixels/sec, which limits the imager frame rate (the rate at which all the imager pixels can be read-out) to 20 Hz for a 1000.times.1000 pixel imager with a single output port.
The inventors knew at the time of their respective inventive contributions of a newer type of optical interferometer using a plane array of CTDs constructed on a substrate of semiconductive material, developed by J. Chovan, a coworker of theirs. With the Chovan type of optical interferometer, the spatial frequency of the pixels as defined by the spacing of the CTDs in parallel rows normal to the fringe pattern is arranged to be only two times the spatial frequency of the fringe pattern, and accommodations for variations in laser pulse amplitudes can be made. Considering the successive fields of pixels generated by the array of CTDs to be consecutively numbered modulo two, the odd-numbered fields of pixels are generated by the fringe pattern between the optical signal and an in-phase coherent optical carrier, and the even-numbered fields of pixels are generated by the fringe pattern between the optical signal and a quadrature coherent optical carrier. The computation of phase variations can be done on the basis of successive overlapping frames identified by respective ones of consecutive ordinal numbers, each odd-numbered frame consisting of an odd-field image succeeded by an even-field image, each even-numbered frame consisting of an even-field image succeeded by an odd-field image, and each frame overlapping its preceding and succeeding frames each by one field. The direct-current pedestal is removed from each field of pixels by performing a subtraction between each pair of pixels that abut each other in the direction normal to the fringe pattern, resampling the fields without DC terms and with the same degrees of attendant phase shift. The resultant in-phase and quadrature-phase field images without direct-current pedestals are correlated in an initial correlation procedure, by multiplying together correspondingly located pixels in each pair of successive fields in a frame, to generate a new succession of image frames, each consisting of but a single field. In this new succession of image frames, pixel values vary in response both to the phase-modulation of the optical signal and the amplitude of the coherent optical signal and the coherent optical carriers used to form the hologram in the focal plane. The variation of pixel values with the product-of-amplitudes term is removed in an amplitude-normalization procedure, using information concerning the amplitudes of the direct-current pedestals removed from the original in-phase and quadrature-phase field images. Amplitude normalization is done either before or after a final correlation procedure in which the pixel values of each of the image frames in the new succession are summed together, which final correlation procedure is done to increase the signal-to-noise ratio of the optical phase detection results.
Before Chovan's newer type of optical interferometer was invented, frame-rates above 10 kHz had only been achieved in optical interferometers using solid-state imagers by:
(a) limiting the pixel resolution, (e.g., using 128.times.1 pixel line arrays) and/or
(b) providing multiple parallel read-out ports from the imager. For the optical interferometers the inventors and their co-workers including Chovan have been called upon to develop, even multi-port imagers are unfeasible. To read out a 1000.times.1000 pixel imager at a 100 kHz frame-rate, 5000 ports running at 2.times.10.sup.4 pixels/sec would be required. Wiring these ports from the imager and providing the digital hardware to support 5000 high-speed processing channels is too costly in terms of size, power, and packaging complexity.
Chovan in developing his new type of optical interferometer had discerned that if pixel-by-pixel correlation procedures can be done parallelly within the solid state imager, rather than sequentially outside the imager, and if the subsequent summation procedure can then be done within the imager as well, then the number of samples of output signal from the imager can be reduced by a factor equal to the number of photosensors in the imager. That is, by a factor of a million for a 1000.times.1000-pixel imager. Furthermore, the parallel processing of photosensor data avoids the need for pixel-scanning generators, which generators and connections from them to individual photosensors take up considerable area on the substrates of prior-art solid state imagers.
Some practical difficulties have been encountered in developing Chovan's new type of optical interferometer. It has been difficult to find a good design for analog multipliers that permits them to be constructed on the imager substrate and integrated with respective photosensors. The amplitude normalization procedure to remove dependency of phase response to the amplitudes of the illuminating and reference beams is cumbersome.
The invention concerns an optical interferometer that does parallel processing of pixel data within the solid state imager. Processing of the in-phase and quadrature-phase images with direct-current pedestals removed is done differently than in the Chovan imager, so as to avoid actually having to multiply correspondingly located pixels in each pair of successive fields together. This better facilitates the pixel processing being done in the charge domain in circuitry constructed on the same substrate as the planar array of CTDs.